1. Field of the Invention
This invention is concerned with seismic streamer cables that are towed under the water for marine geophysical exploration. It is particularly concerned with removal of mechanically-induced noise. Related art might be found in class 367/15 or 367/20.
2. Discussion of the prior Art
In marine geophysical exploration, an acoustic source, such as an air gun, generates an impulse at or near the surface of the water. The seismic waves propagate from the source, through the water and into sub-ocean earth layers, whence they are reflected back to the surface. A plurality of hydrophones, mounted in a streamer cable detect the returning reflected acoustic waves, convert them to electrical signals, and transmit the signals to a signal utilization device aboard a towing vessel, through suitable transmission channels in the streamer cable. The signal transmission channels may consist of metallic conductors, optical fibers or a telemetric system.
Typically, a marine streamer cable consists of an elongated, flexible, tubular plastic jacket, perhaps 6.5 cm in diameter and several millimeters thick. The cable is divided into sections, each about 100 meters long. Each section may include as many as 100 hydrophones, divided into groups of three to ten hydrophones per group. Fifty or more sections may make up the complete streamer cable. Up to three stress members, which may be of stainless steel or of an aramid fiber, are threaded through each section to absorb the towing strain which may be several thousand kilograms. Cylindrical bulkheads, such as are described in U.S. Pat. No. 4,296,481, are disposed internally of the jacket at convenient intervals to preserve the structural integrity thereof. The bulkheads may be perforated for receiving the stress members and signal transmission channels therethrough. The sections are sealed at each end by connectors for mechanically and electrically interconnecting the cable sections to make up the complete streamer cable. Each section is filled with a floatation fluid, typically an aliphatic hydrocarbon such as a light odorless kerosene, to render the cable sections neutrally bouyant in the water.
In use, the stream cable is secured to a suitable ship by a lead-in cable. The ship tows the cable through the water at up to six knots along an assigned line of survey. Usually, the cable is towed at 15 to 20 meters beneath the water surface and is held at the desired depth along its length by means of depth-sensitive paravanes. When not in use, the cable is wound onto a large cable reel at the stern of the ship. Also aboard the ship, there are installed data-processing devices and archival-storage means for receiving the processed data.
In their passage through the water, both cable and ship generate a considerable amount of self-noise. Cable noise is minimized by streamlining the cable. Residual, horizontally-propagating propagating acoustic noise, due in part, for example, to the ship's screw, is removed by spatial filtering such as is taught by C. W. Kerns in U.S. Pat. No. 3,335,401. Acceleration noise, due to small vertical motion of the cable, may be removed by use of well-known acceleration-cancelling hydrophones.
Lurching of the ship, as it moves along the water surface in inclement weather, applies severe linear mechanical accelerations and jerk to the cable. Those mechanical motions create tube or bulge waves in the cable. Bulge waves propagate through the floatation fluid in a waveguide mode and stress the jacket by creating alternate mechanical bulging and necking of the flexible jacket. That action introduces severe, high-amplitude noise to the desired seismic reflection data. That is because the mechanical stressing of the jacket generates acoustic waves in the liquid floatation material. Those acoustic waves are detected by the hydrophones just as if they were valid seismic signals.
V. R. Kurka et al. in U.S. Pat. No. 4,281,402, provide a fluid spring that is attached between the ship and the seismic cable. The fluid spring minimizes the jerk transmitted to the cable. G. W. Loper, in U.S. Pat. No. 3,369,216 uses a resilient member between the ship and the cable, but he adds an inertia element to the cable, hopefully to reduce undesired motion. Although the above devices are useful, I have found that they are not completely effective.
A different approach was taken by V. Gelfand, in U.S. Pat. No. 4,694,436. He provides bulkheads having frustoconical end faces. The tapered end faces radially reflect undesired tube or bulge waves out of the jacket along the normal to the longitudinal axis. That idea is ingenious, but I have found that, because there is little difference between the acoustic impedance of the floatation fluid and that of the plastic bulkheads, the reflection coefficient of the bulkheads is small. For that reason, only a small portion of the undesired mechanical interference is removed by specular reflection from the conical end faces.
Known methods for reducing noise due to severe accelerations and jerk are, at best, only partially effective. I propose, therefore, to accept the presence of mechanically-generated noise. I will provide means for measuring and dynamically removing noise, due to mechanical stress, from desired seismic reflection signals.